The present invention relates generally to optical signal sources for use in optical signal transmission systems, and more particularly to an optical signal source which utilizes a feedback or feedforward technique to provide improved linearity and noise performance without unduly increasing the cost of the source.
In many optical signal transmission applications, it is important for the optical source to exhibit excellent linearity and noise performance. One such application is in high-quality analog optical fiber systems for providing Community Antenna Television (CATV) service. In such a system, an analog CATV signal including several channels of video is modulated onto one or more optical carrier signals for distribution via optical fiber from a cable system head end. Conventional systems for transmitting analog CATV signals over optical fiber generally require highly-linear, low-noise lasers and/or optical modulators. These types of optical signal sources are usually very expensive, and their excessive cost has been a significant impediment to the widespread deployment of fiber-to-the-home transmission systems. The expensive optical signal sources include, for example, analog-grade distributed feedback (DFB) lasers, as well as continuous wave (CW) lasers, such as yttrium aluminum garnet (YAG) and yttrium lanthanum fluoride (YLF) lasers, which require external modulators.
Conventional optical transmission systems are described in, for example, U.S. Pat. No. 5,548,436 entitled xe2x80x9cOptical Transmission Systemxe2x80x9d and issued Aug. 20, 1996 to Ramachandran et al., U.S. Pat. No. 5,457,557 entitled xe2x80x9cLow Cost Optical Fiber RF Signal Distribution Systemxe2x80x9d and issued Oct. 10, 1995 to Zarem et al., U.S. Pat. No. 5,450,508 entitled xe2x80x9cApparatus and Method for Optical Fiber Alignment Using Adaptive Feedback Control Loopxe2x80x9d and issued Sep. 12, 1995 to Decusatis et al., and U.S. Pat. No. 5,384,651 entitled xe2x80x9cOptical Transmission Systemxe2x80x9d and issued Jan. 24, 1995 to Van de Voorde et al. Unfortunately, none of these conventional systems are configured to permit a low-cost optical source, such as a digital-grade DFB laser diode or a Fabry-Perot laser diode, to be used in place of an expensive highly-linear, low-noise optical source.
A number of techniques for improving the linearity or noise performance of an optical source are known. These techniques include the use of pre-distortion circuits, and feedforward approaches in which a second optical source is used. However, conventional pre-distortion circuits generally correct for only source nonlinearity, and an alternative mechanism is therefore required to correct for the source noise. Also, pre-distortion circuits generally require numerous critical adjustments which increase the cost and complexity associated with the source. The second optical source used in conventional feedforward approaches not only involves considerable additional cost and complexity, but introduces wavelength control issues.
It is therefore apparent that a need exists for techniques which provide the linearity and noise performance required in analog signal transmission over optical fiber and other important applications, without the excessive cost and complexity associated with conventional highly-linear, low-noise optical sources.
The present invention improves the linearity and noise performance of an inexpensive optical source using feedback or feedforward techniques. In accordance with a first aspect of the invention, an inexpensive optical source, such as a digital-grade DFB laser diode or a Fabry-Perot laser diode, has its noise and distortion substantially eliminated by feedback. The optical signal source is provided with a feedback path which corrects for nonlinearity and noise-induced variations in the source output. A modulation signal, such as a broadband analog CATV signal, is applied to the optical source along with a broadband feedback signal. The broadband feedback error signal is generated using a high-speed photodiode in the feedback path to detect a portion of the output optical signal generated by the optical source. The broadband feedback error signal is passed through a delay compensation filter to correct for group delay variations across its bandwidth, and then applied to an input of the optical source to correct source nonlinearity and noise. This aspect of the invention is also particularly well-suited for use with digital quadrature-amplitude modulation (QAM) signals, digital n-level vestigial sideband (VSB) signals, and other digital modulation signals which have a non-constant modulation envelope.
In an illustrative embodiment of the invention using the above-described feedback technique, a high-speed photodiode is utilized in a closed feedback loop as a monitoring device to monitor the output of an inexpensive laser diode. The high-speed photodiode is generally substantially more linear than the inexpensive laser diode, and therefore may be used to generate an error signal which can be used to improve the linearity of the laser diode. The high-speed photodiode may have a gain-bandwidth product greater than about 500 MHz, such that the operating bandwidth of the laser diode is substantially increased, while its linearity and noise performance are improved. The feedback loop includes a delay compensation filter which compensates for group delay variations in the feedback error signal across a broad bandwidth.
A second aspect of the invention relates to the use of a feedforward technique to improve the linearity and noise performance of an inexpensive optical source. An error signal generated by monitoring the output of the optical source is converted to a frequency band outside of the modulation signal bandwidth before being applied to an input of the optical source. A receiver receiving the optical signal then detects both the converted error signal and the modulation signal, and uses the detected error signal to correct the detected modulation signal for nonlinearity and noise effects of the optical source.
An illustrative embodiment of this second aspect of the invention includes an optical signal transmitter in which the output of the high-speed photodiode is first low pass filtered, and then applied to a first signal combiner which also receives a coupled portion of the input modulation signal. The first signal combiner generates an error signal by subtracting the output of the lowpass filter and the coupled portion of the modulation signal. The error signal is then applied to an input of a mixer which mixes the error signal with a carrier signal generated by a local oscillator in order to convert the error signal to a frequency band outside the modulation signal frequency band. The converted error signal is combined with the modulation signal in a second signal combiner, such that both the modulation signal and the error signal are applied to the optical source. An output optical signal generated by such a source thus serves as an optical carrier for both the modulation signal and the error signal. The resulting optical signal is detected in a receiver, and the modulation signal portion and error signal portion of the detected signal are separately processed to recover the respective modulation and error signals. The error signal is then used in the receiver to correct the effects of source nonlinearity and noise on the modulation signal. An optical signal transmission system in accordance with the invention includes the above-described transmitter and at least one receiver, and an optical distribution network for distributing an optical signal from the transmitter to the receiver. The distribution network may include passive splitters, multiplexers, demultiplexers and other types of optical signal routers. Multiple optical signals may be routed within the transmission system using multiple transmitters in conjunction with well-known wavelength-division multiplexing (WDM) techniques.